The Breakdown...

Before you dive into a lock and hammer away at the pins, you should develop a general understand of what happens inside the lock as it is opened by a key, or what happens when it doesn't open and why. A largely important skill that lock picking calls upon frequently is the ability to visualize something as it is happening. You then use that visualization to make predictions and decisions about what to do next to successfully pick the lock. So here it goes, listen closely.

A pin tumbler lock contains pins that when not raised to the correct levels by a key, bind inside the lock between the plug and the hull (the locks body). In a four pin lock, there are 8 pins in total, separated into four sets of two pins. In these sets of two pins, the pins are stacked on top of each other. The pin on the bottom is called the key pin, and the pin on top is the driver pin. When the correct key is used, the gaps between all the pins line up with the shear line, the plug will be free to rotate and unlock. When the wrong key is used, the pins will jam and bind at the shear line and the plug will not turn.

Here is a front view of the lock for better understanding.

Try to visualize the corresponding parts of the lock in both of the views presented. What does it look like when the a key is inserted into this lock from both of the views? When the correct key is inserted into this lock, the springs compress under the upward force of the key and driver pins being raised for the key and the gap between the key and driver pin will align with the shear line, allowing the locks plug to turn and open the lock. For a better grasp on what happens to the pins when the correct key is inserted and turned, see the 3D image below. It shows the driver pins resting on top of the plug at the shear line and the key pins resting on the key at their correct levels. Note: When you pick a lock open, the key picks rest on what would be the wards (the teeth like protrusions in the key way, directly under the key pin in the diagram to the right)

As you can see on the left, when the correct key is inserted, the driver pins (blue) are able to clear the shear line, free of any bind, and allow the plug to rotate to the unlocked position.